This invention relates to an online monitor for the continuous determination of the status of a polymer containing process stream through the measurement of polymer molecular weight and/or size and the measurement of composition and concentration of selected species such as monomers and endgroups. The monitor is used, inter alia, to detect the reaction endpoint of a polymer manufacturing process, to monitor polymer quality in a polymer containing process stream, or to perform polymer characterization in an industrial or laboratory setting.
Polymers are formed by a number of reactions, all of which involve the addition or condensation of monomers or other polymer blocks onto growing chains of repeating units. To illustrate, the formation of a polyester is depicted in the following reaction.
nHOROH+nHOOCRxe2x80x2COOHxe2x86x92Hxe2x80x94(xe2x80x94OROxe2x80x94COxe2x80x94Rxe2x80x2xe2x80x94COxe2x80x94)nxe2x80x94OH+(2nxe2x88x921)H2O
The degree of polymerization is represented by the number of repeating units in the chain, which is the integer n in the above reaction. Whether a polymer chain is formed by step-growth polymerization or chain-growth polymerization, the resulting polymers consist of a mixture of polymer molecules with a distribution of molecular weights. The average molecular weight and molecular weight distribution and/or the average size and size distribution of a polymer can be determined by gel permeation chromatography (GPC). For simplicity in the following discussion, the term xe2x80x9cMWRxe2x80x9d will be taken to mean the following: xe2x80x9cweight average molecular weight or molecular sizexe2x80x9d. Thus, the polymer MWR can be determined by GPC. Basic concepts in polymer science and technology are thoroughly reviewed by J. P. Flory, in Principles of Polymer Chemistry, Cornell University Press, 1953.
Polymer MWR can also be determined by techniques such as light scattering, viscosity, osmometry and freezing-point depression (unless noted otherwise, the term light scattering refers to steady state light scattering, as distinguished from dynamic light scattering.). For example, viscosity can be used to determine the polymer MWR of solutions of such polymers as polyesters and proteins or DNA. However, for a dispersed two-phase mixture such as an interfacial process polycarbonate reaction mixture, the solution viscosity is also dependent on the water/organic solvent ratio, the temperature, and the reactor agitation rate, which introduces inaccuracies into this method.
For biological polymers, determination of polymer MWR by viscosity measurement is not preferred. DNA is easily damaged and broken by the type of handling normally associated with viscosity measurements. For protein solutions, viscosity is only accurate for proteins in a random coil configuration. Secondary structure and the degree of denaturation of the protein affect the viscosity of the protein solution, which also depends on such factors as the solution pH, temperature, shear forces, intra- and intermolecular bonding and other factors, making viscosity a highly unreliable method for many types of biological polymers. The molecular weight of biopolymers has been determined in the laboratory by techniques such as SDS-gel electrophoresis, density gradient sedimentation, thin-layer gel chromatography, and viscoelasticity (relaxation time).
Polymer MWR is one of the most important factors that affect polymer properties. For many engineering thermoplastic polymers, as the polymer MWR increases, the mechanical properties of the polymer improve. For example, tensile strength, impact resistance, ductility, and other physical properties of the polymer are all improved with increasing polymer MWR. However, as the polymer MWR increases, the melt viscosity also significantly increases. When the melt viscosity becomes too high, melt processing the polymer becomes difficult or nearly impossible.
In polymer synthesis, the endpoint of a polymerization reaction can be defined as the point at which the polymer meets the desired specifications for all intrinsic polymer properties such as polymer MWR, dispersity, residual endgroup concentration(s), and residual monomer composition and concentration(s). To ensure that a polymerization reaction achieves its endpoint, one should have timely information about these intrinsic polymer properties, or the reaction process conditions such as pH, viscosity, temperature or pressure that are related to the intrinsic polymer properties, or a combination of both. The apparatus of this invention allows online monitoring of the following intrinsic polymer properties: polymer MWR and the concentration(s) of residual monomer(s) and/or endgroups.
In a polymerization reaction, polymer MWR is often the most important intrinsic polymer property to be achieved. In many cases, polymer MWR alone is sufficient to determine the polymerization reaction endpoint. However, in other cases, the reaction endpoint is defined by a specified polymer MWR and by other parameters such as the concentrations of residual monomer and/or endgroups. In such cases, the reaction endpoint must be determined with polymer MWR monitoring plus one or more additional measurements.
For example, for an interfacial polycarbonate manufacturing process, both polymer MWR and residual bisphenol-A (BPA) concentration are important quality parameters. Although a low level ( less than 100 ppm) of BPA monomer in a polycarbonate reaction mixture has no significant effect on the polymer MWR, the presence of this level of monomer can influence the polymer quality and possibly limit its use in certain applications. Therefore, it is often desirable to include both polymer MWR and residual BPA concentration in the definition of the reaction endpoint for a polycarbonate polymerization reaction.
There are many physical and chemical properties of the reaction mixture besides the polymer MWR that change during the course of a polymerization. Current methods for detecting the reaction endpoint are based on one or more of these properties. For example, there are techniques that are based on the heat released during the reaction, decreases in the concentration of monomer, increases in the concentration of the byproducts of the reaction, the occurrence of wasteful side reactions, pH changes, droplet size (in interfacial polymerizations), decreases in end group concentrations, colorimetric assays, and light transmission through the reaction mixture.
For example, several techniques have been developed for detecting the reaction endpoint in interfacial polycarbonate polymerization, as reviewed by Silva and Fyvie in U.S. Pat. No. 5,114,861, which is incorporated herein by reference. These techniques include monitoring the heat release per unit phosgene delivered and monitoring the carbonate ion level in the reaction mixture. Both of these methods are based on detecting the effects of phosgene hydrolysis, a wasteful side reaction that occurs primarily after the polymerization is substantially complete. These techniques have two principal drawbacks. First, they require significant phosgene hydrolysis to occur before a clear endpoint signal can be detected. This means that significant losses in both raw materials and time must occur before the reaction is terminated. Secondly, significant phosgene hydrolysis can occur prior to the true reaction endpoint, due for example to an incorrect catalyst level. This can cause a false endpoint, which would lead to premature termination of the reaction, resulting in low quality polymer.
Attempts have also been made to monitor droplet size and the related dispersion properties by acoustic, focused beam reflectance, and dynamic scattering techniques. These methods have not succeeded because droplet size relates not only to the polymer MWR, but also many to other operational variables such as temperature, agitation rate, the volume ratio of aqueous and organic solvents and others. These factors can not always be completely controlled, which can lead to erroneous results.
U.S. Pat. No. 5,114,861 describes a method of reaction endpoint detection for interfacial polycarbonate polymerization reactions which measures the extent of apparent light transmission. This method is based on the effects of polymer MWR and endgroup concentrations on the polymer phase average droplet size, which influences the extent of apparent light scattering. Although this technique does not require significant phosgene hydrolysis to be effective, the detector signal is somewhat sensitive to the operating and thus these conditions must be compensated for or carefully controlled.
Inferring a reaction endpoint from process measurements which are indirectly related to the true polymer MWR is by its very nature imprecise. The measured properties such as the rate of heat release depend on a number of process variables in addition to the polymer MWR. In addition, the prior art methods of polymerization reaction endpoint detection are highly process- and equipment-specific. These methods, therefore, while somewhat effective for specific applications, are subject to serious errors if process or equipment changes are made.
Ensuring consistent polymer quality in a polymer containing process stream is also highly desirable. For example, if the polymer MWR of a polymer containing process stream that feeds a resin dryer falls below a specified level, dryer fusion can result, in which the polymer particles become plasticized as they are heated above their glass transition temperature. This results in agglomeration into large viscous polymer masses that can plug process equipment, which leads to extensive down time. A method of reliably and quickly monitoring the polymer MWR of a polymer containing process stream is thus essential to ensuring high productivity and consistent product quality.
Using light scattering and concentration measurements to determine polymer MWR in combination with GPC is acceptable in a laboratory environment. However, this method has not been applied to monitor the polymer MWR online in a manufacturing process because light scattering measurements require low polymer concentrations in solution and GPC leads to unacceptable measurement delays. In addition, both light scattering and GPC require that the analyte solution be free of particulates and bubbles, and exist as a single phase. Unfortunately, most manufacturing processes introduce particulates and bubbles into polymer containing process streams, and many processes involve multiple phases. Removal of these interfering materials typically requires procedures that introduce delays that are not consistent with the response time requirements of a monitor for monitoring and control of a rapidly changing system. None of the prior art methods of polymer process monitoring is adaptable to an online, continuous, system which produces no waste stream, does not result in loss of polymer due to sampling the polymer containing process stream, and is insensitive to changes in process conditions such as the agitation rate.
Consequently, there is a great need for a flexible, online monitor to monitor the status of both polymer containing process streams and polymerization reactions. Such a monitor must be applicable to a variety of polymer processes and provide continuous process data. When used as a reaction endpoint detector, such a monitor would allow control of the termination of the reaction at the correct time and not require the addition of reagents to the analyte stream, thereby allowing the analyte stream to be returned to the process. Such a device or method would enable continuous online monitoring of reaction status or endpoint for a variety of batch or continuous polymerization processes to ensure consistent polymer quality, reduced raw materials and energy usage, and increased productivity. Such a device would also enable continuous online monitoring of a polymer containing process stream and enable rapid quality control checks.
In one aspect, the present invention provides an online polymer monitoring apparatus for determination of polymer properties, comprising:
a) a flow through detector means having at least one inlet and one outlet, comprising at least one light scattering detector, at least one polymer concentration detector, and optionally additional detectors;
b) a sampling and delivery system for collecting a representative sample from a polymer containing process stream, having
i) a first flow means having a first end connected to the polymer containing process stream and a second end connected to the detector means inlet;
ii) optionally a dilution means comprising a solvent source and a second flow means having a first end connected to the solvent source and a second end connected to the first flow means for delivering solvent to the first flow means;
iii) optionally a sample preparation means in association with the first flow means, which prepares the analyte solution from the representative sample by separating and/or filtering interfering materials;
iv) optionally, a means associated with the first flow means for controlling the flow of the representative sample from the polymer containing process stream to the detector means;
v) optionally, a third flow means for directing material from the outlet of the detector means to the polymer containing process stream; and
vi) optionally, a circulation loop comprising a fourth flow means between the polymer containing process stream and the first flow means through which polymeric material is moved out of the polymer containing process stream and returned to the polymer containing process stream driven by a pumping and/or pressure differential mechanism; and
c) means associated with the flow through detector means for calculating the polymer properties in the representative sample in response to data obtained by said detector means.
In another aspect, the present invention provides an online method for monitoring the status of a polymer containing process, which comprises:
a) collecting and preparing a sample online from a polymer containing process stream;
b) analyzing the collected and prepared sample by obtaining at least one of the following: the concentration responses of at least one polymer in said sample, the polymer light scattering responses of said sample, and/or the detector responses of the selected species in said sample;
c) inferring a polymer property from the results of step b).